Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
The conversion of oxygenates to olefins (OTO), particularly the conversion of methanol to olefins (MTO), in a hydrocarbon conversion apparatus generates and deposits carbonaceous material (coke) on the molecular sieve catalysts used to catalyze the conversion process. Excessive accumulation of these carbonaceous deposits will interfere with the catalyst's ability to promote the reaction. In order to avoid unwanted build-up of coke on molecular sieve catalysts, the OTO and MTO processes incorporate a second step comprising catalyst regeneration. During regeneration, the coke is at least partially removed from the catalyst by combustion with oxygen, which restores the catalytic activity of the catalyst and forms a regenerated catalyst. The regenerated catalyst then may be reused to catalyze the conversion of methanol to olefins.
In conventional regeneration vessels, coked catalyst is directed from a reactor to a catalyst regenerator. In a catalyst regenerator, a regeneration medium, usually oxygen, enters the regenerator, and coke is removed from the coked catalyst by combustion with the regeneration medium to form regenerated catalyst and gaseous byproducts. The bulk of the regenerated catalyst from the regenerator is returned to the reactor. The gaseous byproducts are forced out an exhaust outlet oriented in the upper section of the catalyst regenerator.
The combustion of the carbonaceous deposits from molecular sieve catalyst compositions during catalyst regeneration is an exothermic process. The exothermic nature of catalyst regeneration presents a problem in OTO regeneration systems because the amount of coke formed on the molecular sieve catalyst compositions utilized in OTO reaction systems preferably is maintained at higher levels than in conventional FCC processes in order to maintain a high prime olefin (ethylene and propylene) selectivity. As a result, the amount of heat liberated from the OTO molecular sieve catalyst compositions during catalyst regeneration is significantly greater than the amount of heat liberated from the regeneration of catalysts in FCC processes.
The tremendous amount of heat liberated during the regeneration of heavily coked catalyst particles, such as coked OTO catalyst particles, may exceed the metal tolerances of the metals used to form the catalyst regenerator, particularly of the separation vessels, e.g., cyclone separators, contained therein as well as the conduits used to transport regenerated catalyst back to the hydrocarbon conversion apparatus. The creation of localized “hot spots” in catalyst regenerators also poses a significant problem in that catalyst is not regenerated uniformly throughout the regeneration zone. The heat also can damage the catalyst particles themselves. As a result, improved processes are sought for regenerating highly coked catalyst particles, such as coked catalyst particles derived from OTO reaction systems, while maintaining desirable temperature characteristics in the OTO catalyst regenerator.